I bought an x-valve off of ebay around 2008 that was pretty abused, but I like it because it is all worn-in, runs loose, and likes to RT just a hair. It occasionally had had that goofy, tiny leak after the on/off top that you think is the power tube. It would stop with oil. It finally wouldn't seal up anymore, and after checking the lvl10, I was left with that little beast.
I had received a spare ULT o-ring in a box of parts VF-XX had traded me a few years ago. I was very happy to be able to locate that tiny thing in my garage, in a desk top full of boxes of parts. I pulled the other parts off the ULT and the bottom o-ring, clamped the pin in a brass hand vice, mounted the hand vice in the bench vise, and was preparing to pull out my MAPP torch to heat the bottom of the pin. I grabbed the pin bottom with my fingers to make sure it was held firm, and it turned (!?). After a little wiggling around, the bottom spun freely and pulled right off! I cleaned the old glue off, put on the new-ish o-ring and a little blue loctite, then stuck it together to set up. It's working great. The old o-ring didn't look bad, but it fit loose and had become hardened.
I love it when the parts all jump together!